1. Field of the Invention
The present invention relates to a lens and a method of molding the lens.
2. Description of the Related Art
A lens is used in an optical system of an imaging unit that is mounted on a mobile phone or a digital camera. The design of a small lens has proceeded with the downsizing and multi-functionalization of a mobile phone or the like.
Edge portions, which are mechanically supported by a holding frame and a lens barrel or maintain an interval between adjacent lenses in the direction of an optical axis when a lens is supported by the holding frame and the lens barrel and mounted on an imaging unit, are provided at the outer periphery of optical functional surfaces, which have a function of refracting light, of the lens. Further, the shape of an outer peripheral portion of the lens including these edge portions affects the eccentricity of the lens or the interval between the lenses in the direction of the optical axis. For this reason, not only the shape of the optical functional surface but also the edge portion that is provided at the outer peripheral portion of the optical functional surface is required to be accurately formed in the molding of the lens.
Here, injection molding in which molten resin is injected into a cavity partitioned in a mold so as to fill the cavity, and compression molding in which a resin is compressed by a mold and is molded in the shape of a molding surface formed in the mold are known as a method of manufacturing a resin lens.
In general, injection molding is a major method of molding the current resin lens, and is suitable for the continuous molding of multiple units. However, since the cavity is filled with molten resin, the resin does not smoothly flow when the lens is thin. For this reason, imperfect filling or the deterioration of the accuracy of a surface caused by the reduction of molding pressure may occur. Further, since molten resin flows in from a gate, strain caused by internal stress or the like is generated near the gate. As a result, the optical anisotropy (birefringence) of a lens may occur.
Meanwhile, compression molding is inferior to injection molding in terms of mass production, but can compensate for the shortcomings of injection molding. That is, since compression molding is a molding method in which a lump (preform) of a softened resin is squashed by a mold, the compression molding is not affected by the flow of a resin and is suitable for making a molded body thin as compared to injection molding. Further, optical anisotropy caused by the flow of a resin does not occur in compression molding unlike in injection molding. However, since a preform is squashed in compression molding, a portion, which absorbs an influence of variation in the volume of the preform, is required in compression molding. In the case of a lens, not only the accuracy of a surface but also the accuracy of a center thickness is an important parameter. As a result, since variation in the volume of the preform is absorbed by an external portion, it is difficult to improve the dimensional accuracy of an outer radial portion including the external portion.
There is a method disclosed in JP2007-22905A as a method of accurately forming the external shapes of an optical functional surface and a lens. JP2007-22905A discloses a molding method in which the compression molding of an optical element is performed by a mold and a frame body is bonded to the optical element with reference to the position of the mold while the optical element is held without opening the mold.